A typical example of the resonant-tunneling hot electron transistor is disclosed by Yokoyama et al. in "A New Functional, Resonant-Tunneling Hot Electron Transistor (RHET)", Japanese Journal of Applied Physics, Vol. 24, No. 11, November, 1985, pages L853 to L854. The resonant-tunneling hot electron transistor disclosed by Yokoyama has an unique energy band diagram shown in FIG. 1 in which a quantum well 1 is formed in the quantum well structure intervening between the n-type emitter region and the n-type base region. Between the n-type base region and the n-type collector region is formed the collector barrier region which provides a potential barrier 2 with respect to the carriers traveling over the n-type base region. E1 indicates the energy of resonant state formed in the quantum well.
In the thermal equilibrium state, no carrier injection takes place from the quantum well to the base region. However, when an appropriate biasing voltage is applied between the n-type emitter region and the n-type base region, a resonant tunneling phenomenon takes place, so that electrons 3 are injected into the n-type base region. Each of the electrons injected into the n-type base region travels in a ballistic or near-ballistic manner toward the n-type collector region. However, the electrons experience various types of scattering such as an inter-valley scattering during the travel in the n-type base region, so that electrons are scattered and accordingly lose the respective kinetic energies This results in that most of the electrons can not excess the potential barrier 2 formed between the n-type base region and the n-type collector barrier region. For reduction of the undesirable scattering, it is necessary to form a thin base region smaller in thickness than the mean free path of the electron. However, the base region is hardly reduced to a thickness less than the mean free path of the electron because the base resistance is increased due to reduction in sheet carrier density. Moreover, another problem is encountered in the prior-art resonant-tunneling hot electron transistor in formation of base-collector junction. There are trade-offs between the amount of electrons excessing the potential barrier 2 and the base resistance or the easy formation. Then, the resonant-tunneling hot electron transistor merely realizes a current gain ranging between value 1 and value 5. Moreover, the collector barrier region is insufficient to block the thermo-electrons at room temperatures, and, for this reason, a large amount of leakage current takes place between the base region and the collector region. In other words, the resonant-tunneling hot electron transistor hardly operates in the room temperatures.
Another example is disclosed by Capasso et al. in "QUANTUM-WELL RESONANT TUNNELING BIPOLAR TRANSISTOR OPERATION AT ROOM TEMPERATURE", IEEE Electron Device Letters, Vol. EDL-7, No. 10, October 1986, pages 573 to 576. The resonant-tunneling bipolar transistor disclosed by Capasso et al. is also provided with a quantum well structure for carrier injection, and a negative differential resistance takes place in the base-emitter I-V characteristics as similar to the resonant-tunneling hot electron transistor disclosed by Yokoyama. However, no collector barrier region is formed between the base region and the collector region. Instead of the collector barrier region, a reverse bias p-n junction is used to block the thermo-electrons as shown in FIG. 2. Although the injected electrons are scattered during the travel over the p-type base region and, accordingly, lose the respective kinetic energy, a substantial amount of electrons reach the collector region because no potential barrier is formed between the p-type base region and the n-type collector region. These electrons are conducive to improvement in current gain or current driving capability. However, a problem is encountered in the resonant-tunneling bipolar transistor disclosed by Capasso in the amount of kinetic energy applied to each electron injected into the base region. This is because of the fact that not only base-collector junction but also emitter-base junction are of the p-n type. Each electron injected beyond the emitter-base junction of the p-n type accepts a relatively small amount of kinetic energy which results in that each electron consumes a relatively long time period until reaching the collector region. In detail, the switching speed of the transistor is dominated by the accumulation time period Te of emitter capacitance, the base transit time period Tb and the accumulation and transit time period of collector Tc. The accumulation time period Te and the accumulation and transit time period Tc of the resonant-tunneling bipolar transistor are on respective levels with the resonant-tunneling hot electron transistor. However, the base transit time period Tb is less than that of the resonant-tunneling hot electron transistor, so that the resonant-tunneling bipolar transistor is not on a level with the resonant-tunneling hot electron transistor disclosed by Yokoyama in view of switching speed.
Another prior-art bipolar transistor is disclosed by Matsumoto et al. in "GaAs Inversion-Base Bipolar Transistor (GaAs IBT)", IEEE ELECTRON DEVICE LETTERS, Vol. EDL-7, No. 11, November 1986, pages 627 and 628. The GaAs inversion-base bipolar transistor has a base formed by a two-dimensional hole gas.